Catalysed soot filter for treating the exhaust gas of a compression ignition engine

ABSTRACT

A catalyzed soot filter comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from a compression ignition engine disposed on a filtering substrate, wherein the oxidation catalyst comprises: a platinum group metal (PGM) component selected from the group consisting of a platinum (Pt) component, a palladium (Pd) component and a combination thereof; an alkaline earth metal component; a support material comprising a modified alumina incorporating a heteroatom component.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 14/086,062 filed Nov. 21,2013, which claims priority benefit to Great Britain Patent ApplicationNo. 1220912.8 filed on Nov. 21, 2012 and to U.S. Provisional PatentApplication No. 61/728,834 filed no Nov. 21, 2012, and to Great BritainPatent Application No. 1308934.7 filed on May 17, 2013, all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to catalysed soot filter for a compressionignition engine, which catalysed soot filter comprising an oxidationcatalyst disposed on a filtering substrate and to an exhaust systemcomprising the catalysed soot filter, to a compression ignition enginecomprising the exhaust system and to a vehicle comprising the exhaustsystem. The invention also relates to use of the catalysed soot filterand to a method of treating an exhaust gas from a compression ignitionengine.

BACKGROUND TO THE INVENTION

Generally, there are four classes of pollutant that are legislatedagainst by inter-governmental organisations throughout the world: carbonmonoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NO_(x))and particulate matter (PM). As emissions standards for permissibleemission of pollutants in exhaust gases from vehicular engines becomeprogressively tightened, there is a need to provide improved catalyststhat are able to meet these standards and which are cost-effective.

For compression ignition engines, such as diesel engines, a catalysedsoot filter (CSF) is typically used to treat the exhaust gas produced bysuch engines. CSFs generally catalyse the oxidation of (1) carbonmonoxide (CO) to carbon dioxide (CO₂), (2) HCs to carbon dioxide (CO₂)and water (H₂O) and (3) the oxidation of PM filtered from the exhaustgas. The two most important PM oxidation reactions are oxidation innitrogen dioxide (NO₂+C→NO+CO) and oxygen (O₂+2C→2CO or O₂+C→CO₂).Sources of NO₂ for the former reaction are the engine itself andnitrogen monoxide (also present in the exhaust gas) oxidised either onan upstream substrate monolith comprising a diesel oxidation catalyst(DOC) or on the filter catalyst itself. Exhaust gas temperatures forcompression ignition engines, such as diesel engines particularly forlight-duty diesel vehicles, are relatively low (e.g. about 400° C.) andso one challenge is to develop durable CSF catalyst formulations withlow “light-off” temperatures.

The activity of oxidation catalysts, such as CSFs and DOCs, is oftenmeasured in terms of their “light-off” temperature, which is thetemperature at which the catalyst starts to perform a particularcatalytic reaction or performs that reaction to a certain level.Normally, “light-off” temperatures are given in terms of a specificlevel of conversion of a reactant, such as conversion of carbonmonoxide. Thus, a T₅₀ temperature is often quoted as a “light-off”temperature because it represents the lowest temperature at which acatalyst catalyses the conversion of a reactant at 50% efficiency.

Low Emission Zones (LEZs) are areas or roads across Europe, e.g. Berlin,London, Stockholm, Eindhoven etc., where the most polluting vehicles arerestricted from entering (seehttp://www.lowermissionzones.eu/what-are-lezs?showall=1&limitstart=).There is growing evidence that poor air quality is bad for health andlife expectancy. Nitrogen dioxide is considered to have both short-termand long-term effects on health. It affects lung function and exposureenhances the response to allergens in sensitised individuals. It hasbeen suggested that apparent effects of nitrogen dioxide on health maybe due to particles or to its combination with particles. NO₂ can alsocontribute to reactions causing photochemical smog. EU Air QualityStandards (binding on EU Member States) sets Limit Values for theprotection of human health. The EU Air Quality Standard for inter aliaNO₂ was set from 1 Jan. 2010 at 200 μg/m³ (105 ppb) average over a 1hour period, not to be exceeded >18 times a calendar year; and a 40μg/m³ (21 ppb) average per calendar year.

There is therefore a need in the art for exhaust systems which avoid orreduce NO₂ emission into the atmosphere, particularly for vehiclesaccessing LEZs. These can include factory fit exhaust systems andsystems to be retrofitted to existing vehicles.

WO 00/34632 discloses a system for treating the exhaust gases fromdiesel engines comprising a first catalyst effective to oxidisehydrocarbons, a second catalyst effective to convert NO to NO₂, a trapfor particulates, on which particulates may be combusted in NO₂. Thefirst catalyst can be platinum dispersed on ceria or on a metal oxidewashcoat which incorporates ceria. The Examples explain that: “It isevident that once the HC (represented by C₃H₆) has been removed in thefirst oxidation step the oxidation of NO to NO₂ can take place morecompletely”.

Catalysts that are used to oxidise carbon monoxide (CO), hydrocarbons(HCs) and sometimes also oxides of nitrogen (NO_(x)) in an exhaust gasemitted from a compression ignition engine generally comprise at leastone platinum group metal, such as platinum or palladium. Platinum ismore active than palladium at catalysing the oxidation of CO and HCs inthe exhaust gas from a compression ignition engine and the inclusion ofpalladium in such catalysts was generally avoided because of itssusceptibility to poisoning by sulphur. However, the use of ultra-lowsulphur fuels, the relative cost of palladium to platinum, andimprovements in catalyst durability that can be obtained by inclusion ofpalladium have resulted in catalyst formulations comprising palladium,especially formulations comprising both palladium and platinum, becomingfavoured.

Even though, in general, the cost of palladium has historically beenlower than that of platinum, both palladium and platinum are expensivemetals. Oxidation catalysts that show improved catalytic activitywithout increasing the total amount of platinum and palladium, or thatshow similar catalytic activity to existing oxidation catalysts with alower amount of platinum and palladium, are desirable.

SUMMARY OF THE INVENTION

The inventors have surprisingly found that an oxidation catalyst havingadvantageous activity including stable, relatively low NO oxidationactivity can be obtained when a combination of (i) an alkaline earthmetal component and (ii) an alumina support material that has beenmodified to include a heteroatom component, is included in a catalystformulation comprising at least one of platinum and palladium. Suchcatalysts can be used with advantage in exhaust systems for use in LEZs,where low NO₂ emissions are required. Such catalysts have been found tohave excellent low temperature CO oxidation activity. The catalysts areparticularly effective in converting relatively high levels of CO inexhaust gas produced by the compression ignition engine, particularly attemperatures below 250° C. The catalysts may also show good oxidationactivity towards HCs, particularly unsaturated HCs such as alkenes, atlow temperatures. The relatively low temperature oxidation activity ofthe catalyst renders it particularly suitable for use in combinationwith other emissions control devices in an exhaust system. Inparticular, although NO oxidation is relatively low, the oxidationcatalyst is able to oxidise nitrogen oxide (NO) to nitrogen dioxide(NO₂), which can be advantageous when the oxidation catalyst is upstreamof a selective catalytic reduction (SCR) catalyst or a filter catalysedwith a selective catalytic reduction catalyst.

Therefore, according to a first aspect, the invention provides acatalysed soot filter comprising an oxidation catalyst for treatingcarbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from acompression ignition engine disposed on a filtering substrate, whereinthe oxidation catalyst comprises: a platinum group metal (PGM) componentselected from the group consisting of a platinum (Pt) component, apalladium (Pd) component and a combination thereof; an alkaline earthmetal component; a support material comprising a modified aluminaincorporating a heteroatom component.

Without wishing to be bound by any particular theory, inventors believethat the stable, relatively low NO oxidation activity of oxidationcatalysts for use in the present invention may be due to a combinationof factors, including competitive selective oxidation of CO and HCspecies, i.e. NO oxidation occurs only after CO and HC have beenoxidatively removed; and the oxidation by NO₂ of non-polar (e.g.aliphatic), longer, straight-chain HC species such as dodecane (akin to“classical” lean NO_(x) catalysis (also known as HC-SCR), i.e.{HC}+NO_(x)→N₂+CO₂+H₂O). Accordingly, it is believed that the oxidationcatalyst for use in the present invention can not only reduce NO₂emissions from the catalysed soot filter per se (by relatively low NOoxidation activity) but also to reduce NO₂ emissions from upstreamcatalysts entering the catalysed soot filter, i.e. the quantity of NO₂exiting the catalysed soot filter according to the invention may be lessthan the quantity entering it.

The initial oxidative activity of a freshly prepared oxidation catalystoften deteriorates until the catalyst reaches an aged state. Repeatedexposure of the oxidation catalyst to hot exhaust gas can causesintering and/or alloying of the platinum group metal (PGM) componentsof the catalyst until it reaches an aged state. This deterioration inactivity can be problematic, particularly when pairing the oxidationcatalyst with one or more other emissions control devices in an exhaustsystem. The oxidation catalyst of the invention may have stable activitytoward oxidising nitrogen oxide (NO) to nitrogen dioxide (NO₂) (i.e. the“fresh” oxidative activity of the catalyst toward NO is the same orsimilar to the “aged” oxidative activity of the catalyst). This isparticularly advantageous for exhaust systems where the oxidationcatalyst is combined with a selective catalytic reduction (SCR) catalystor a filter catalysed with a selective catalytic reduction catalystbecause an exhaust gas having a stable ratio of NO:NO₂ can be passedinto the SCR catalyst or SCR catalysed filter.

A second aspect of the invention relates to an exhaust system for acompression ignition engine comprising a catalysed soot filter accordingto the first aspect of the invention.

In a third aspect, the invention relates to a compression ignitionengine comprising an exhaust system according to the second aspect ofthe invention.

In a fourth aspect, the invention relates to a vehicle comprising acompression ignition engine according to the third aspect of theinvention.

In a fifth aspect, the invention provides the use of catalysed sootfilter according to the first aspect of the invention to oxidise carbonmonoxide (CO) and hydrocarbons (HCs) and for reducing NO₂ emissions inan exhaust gas from a compression ignition engine.

A sixth aspect of the invention relates to a method of treating carbonmonoxide (CO) and hydrocarbons (HCs) and NO₂ in an exhaust gas from acompression ignition engine, which method comprises contacting theexhaust gas with a catalysed soot filter according to the first aspectaccording to the invention

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a catalysed soot filter comprising an oxidationcatalyst comprising an alkaline earth metal component. It hassurprisingly been found that a catalyst having advantageous oxidisingactivity, particularly a low CO T₅₀, can be obtained for catalystformulations comprising an alkaline earth metal component and a modifiedalumina incorporating a heteroatom component.

Typically, the alkaline earth metal component comprises magnesium (Mg),calcium (Ca), strontium (Sr), barium (Ba) or a combination of two ormore thereof. It is preferred that the alkaline earth metal componentcomprises calcium (Ca), strontium (Sr), or barium (Ba), more preferablystrontium (Sr) or barium (Ba), and most preferably the alkaline earthmetal component comprises barium (Ba).

Generally, the alkaline earth metal component comprises a singlealkaline earth metal selected from the group consisting of (Mg), calcium(Ca), strontium (Sr) and barium (Ba). Preferably, the alkaline earthmetal component comprises a single alkaline earth metal selected fromthe group consisting of calcium (Ca), strontium (Sr) and barium (Ba),more preferably strontium (Sr) and barium (Ba), and most preferably thealkaline earth metal component comprises a single alkaline earth metalthat is barium (Ba).

Typically, the amount of the alkaline earth metal component is 0.07 to3.75 mol ft⁻³, particularly 0.1 to 3.0 mol ft⁻³, more particularly 0.2to 2.5 mol ft⁻³ (e.g. 0.25 to 1.0 mol ft⁻³), such as 0.3 to 2.25 molft⁻³, especially 0. 0.35 to 1.85 mol ft⁻³, preferably 0.4 to 1.5 molft⁻³, even more preferably 0.5 to 1.25 mol ft⁻³. Without wishing to bebound by theory, it is believed that the number of alkaline earth metalatoms that are present contributes to the advantageous activity of thecatalyst and that this activity “levels-off” once the number of alkalineearth metal atoms has reached a certain amount. The ability of thecatalyst to oxidise certain HC species and NO can be reduced withincreasing alkaline earth metal content.

Generally, the total amount of the alkaline earth metal component is 10to 500 g ft⁻³ (e.g. 60 to 400 g ft⁻³ or 10 to 450 g ft⁻³), particularly20 to 400 g ft⁻³, more particularly 35 to 350 g ft⁻³, such as 50 to 300g ft⁻³, especially 75 to 250 g ft⁻³.

The oxidation catalyst for use in the present invention, in general,comprises an amount of the alkaline earth metal component of 0.1 to 20%by weight, preferably 0.5 to 17.5% by weight, more preferably 1 to 15%by weight, and even more preferably 1.5 to 12.5% by weight. The amountof the alkaline earth metal component may be from 1.0 to 8.0% by weight,such as 1.5 to 7.5% by weight, particularly 2.0 to 7.0% by weight (e.g.2.5 to 6.5% by weight or 2.0 to 5.0% by weight). The amount of thealkaline earth metal component may be from 5.0 to 17.5% by weight, suchas 7.5 to 15% by weight, particularly 8.0 to 14% by weight (e.g. 8.5 to12.5% by weight or 9.0 to 13.5% by weight).

Typically, the ratio of the total mass of the alkaline earth metalcomponent to the total mass of the platinum group metal (PGM) componentis 0.25:1 to 20:1 (e.g. 0.3:1 to 20:1). It is preferred that the ratioof the total mass of the alkaline earth metal component to the totalmass of the platinum group metal (PGM) component is 0.5:1 to 17:1, morepreferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1. When aplatinum (Pt) component is present, then preferably the total mass ofthe alkaline earth component is greater than the total mass of theplatinum (Pt) component.

The support material typically comprises, or consists essentially of, amodified alumina incorporating a heteroatom component. The heteroatomcomponent that is incorporated into the alumina generally changes thechemical characteristics, physical structure and/or physical propertiesof the material in comparison to alumina itself, and generally also incomparison to a mixture of alumina with the heteroatom component. It isthought that the presence of the heteroatom component modifies theinteraction of the alumina with the alkaline earth component. Themodified alumina is typically alumina present in, or originating from,the gamma form (i.e. γ-alumina).

Typically, the heteroatom component comprises an element selected fromthe group consisting of a lanthanide and any one of Groups 1 to 14 ofthe Periodic Table (the IUPAC nomenclature for numbering the Groups ofthe Periodic Table is used herein, such that Group 1 comprises thealkali metals, Group 4 comprises Ti, Zr etc., and Group 14 comprises C,Si etc.). Preferably, the heteroatom component comprises an elementselected from Group 2 (e.g. Mg, Ca, Sr or Ba), Group 4 (e.g. Ti or Zr),Group 14 (e.g. Si) of the Periodic Table and a lanthanide (e.g. La orCe), such as an element selected from Group 4 (e.g. Ti or Zr), Group 14(e.g. Si) of the Periodic Table and a lanthanide (e.g. La or Ce). Theheteroatom component can be an element, ion or a compound, but it is notalumina and, preferably, it is not a constituent element or ion ofalumina (e.g. oxygen, O²⁻, aluminium or Al³⁺).

The modified alumina incorporating a heteroatom component generallycomprises, or consists essentially of, an alumina doped with aheteroatom component, an alkaline earth metal aluminate or a mixturethereof. It is preferred that the modified alumina incorporating aheteroatom component comprises, or consists essentially of, an aluminadoped with a heteroatom component or an alkaline earth metal aluminate.

When the modified alumina incorporating a heteroatom component isalumina doped with a heteroatom component, then typically the heteroatomcomponent comprises silicon, magnesium, barium, lanthanum, cerium,titanium, or zirconium or a combination of two or more thereof. Theheteroatom component may comprises, or consist essentially of, an oxideof silicon, an oxide of magnesium, an oxide of barium, an oxide oflanthanum, an oxide of cerium, an oxide of titanium or an oxide ofzirconium. Preferably, the heteroatom component comprises, or consistsessentially of, silicon, magnesium, barium, or cerium, or an oxidethereof, particularly silicon, or cerium, or an oxide thereof. Morepreferably, the heteroatom component comprises, or consists essentiallyof, silicon, magnesium, or barium, or an oxide thereof; particularlysilicon, or magnesium, or an oxide thereof; especially silicon or anoxide thereof.

Examples of alumina doped with a heteroatom component include aluminadoped with silica, alumina doped with magnesium oxide, alumina dopedwith barium or barium oxide, alumina doped with lanthanum oxide, oralumina doped with ceria, particularly alumina doped with silica,alumina doped with lanthanum oxide, or alumina doped with ceria. It ispreferred that the alumina doped with a heteroatom component is aluminadoped with silica, alumina doped with barium or barium oxide, or aluminadoped with magnesium oxide. More preferably, the alumina doped with aheteroatom component is alumina doped with silica or alumina doped withmagnesium oxide. Even more preferably, the alumina doped with aheteroatom component is alumina doped with silica. Alumina doped with aheteroatom component can be prepared using methods known in the art or,for example, by a method described in U.S. Pat. No. 5,045,519.

Typically, the alumina doped with a heteroatom component comprises 0.5to 45% by weight of the heteroatom component, preferably 1 to 40% byweight of the heteroatom component, more preferably 1.5 to 30% by weightof the heteroatom component, particularly 2.5 to 25% by weight of theheteroatom component.

When the alumina doped with a heteroatom component comprises, orconsists essentially of, alumina doped with silica, then the alumina isdoped with silica in an amount of 0.5 to 45% by weight, preferably 1 to40% by weight, more preferably 1.5 to 30% by weight (e.g. 1.5 to 10% byweight), particularly 2.5 to 25% by weight, more particularly 3.5 to 20%by weight (e.g. 5 to 20% by weight), even more preferably 4.5 to 15% byweight.

When the alumina doped with a heteroatom component comprises, orconsists essentially of, alumina doped with magnesium oxide, then thealumina is doped with magnesium in an amount as defined above or anamount of 5 to 30% by weight, preferably 10 to 25% by weight.

If the heteroatom component comprises, or consists essentially of, analkaline earth metal, then generally the oxidation catalyst comprises analkaline earth metal component that is separate to, or is not part of,the modified alumina incorporating a heteroatom component. Thus, theoxidation catalyst includes an alkaline earth metal component inaddition to any alkaline earth metal that may be present in the modifiedalumina.

Generally, when the heteroatom component comprises, or consistsessentially of, an alkaline earth metal, then preferably the alkalineearth metal component is different to the heteroatom component. It ispreferred that the heteroatom component and the alkaline earth metalcomponent comprise different alkaline earth metals.

If the heteroatom component of the modified alumina comprises analkaline earth metal, such as when it is a dopant in the alumina dopedwith a heteroatom component or when it is part of the alkaline earthmetal aluminate, then the amount of the “alkaline earth metal component”does not include the amount of any alkaline earth metal that is presentas part of the modified alumina. Similarly, the amount of heteroatomcomponent does not include the amount of the alkaline earth metalcomponent that is present. It is possible to control the amounts of eachcomponent during manufacture of the oxidation catalyst.

The term “alkaline earth metal aluminate” generally refers to a compoundof the formula MAl₂O₄ where “M” represents the alkaline earth metal,such as Mg, Ca, Sr or Ba. Such compounds generally comprise a spinelstructure. These compounds can be prepared using conventional methodswell known in the art or, for example, by using a method described in EP0945165, U.S. Pat. No. 6,217,837 or U.S. Pat. No. 6,517,795.

Typically, the alkaline earth metal aluminate is magnesium aluminate(MgAl₂O₄), calcium aluminate (CaAl₂O₄), strontium aluminate (SrAl₂O₄),or barium aluminate (BaAl₂O₄), or a mixture of two or more thereof.Preferably, the alkaline earth metal aluminate is magnesium aluminate(MgAl₂O₄).

Generally, when the support material comprises an alkaline earth metalaluminate, then the alkaline earth metal (“M”) of the alkaline earthmetal aluminate is different to the alkaline earth metal component. Itis preferred that the alkaline earth metal aluminate and the alkalineearth metal component comprise different alkaline earth metals.

The oxidation catalyst for use in the invention generally comprises atotal amount of support material of 0.1 to 5 g in⁻³, preferably 0.2 to 4g in⁻³ (e.g. 0.5 to 3.5 g in⁻³). When the oxidation catalyst for use inthe present invention comprises a second support material, in additionto the support material comprising the modified alumina, then the totalamount refers to the amount of both the second support material and thesupport material comprising the modified alumina.

The total amount of support material in the oxidation catalyst of thecatalysed soot filter is generally 0.2 to 4 g in⁻³.

When the oxidation catalyst for use in the present invention comprises asecond support material, then typically the amount of the supportmaterial comprising the modified alumina is 0.1 to 3.0 g in⁻³,preferably 0.2 to 2.5 g in⁻³, still more preferably 0.3 to 2.0, and evenmore preferably 0.5 to 1.75 g in⁻³.

In general, the ratio of the total mass of the alkaline earth metalcomponent to the total mass of the support material comprising themodified alumina is 1:200 to 1:5, preferably 1:150 to 1:10, even morepreferably 1:100 to 1:20.

Typically, the support material, particularly the alumina doped with aheteroatom component, is in particulate form. The support material mayhave a d₉₀ particle size of 20 μm (as determined by conventional laserdiffraction techniques). The particle size distribution of the supportmaterial is selected to aid adhesion to the substrate. The particles aregenerally obtained by milling.

Generally, the support material has a specific surface area of 50 to 500m² g⁻¹ (measured by BET in accordance with DIN 66131 or after activationat 550° C. for 3 hours). It is preferred that the support material has aspecific surface area of 50 to 300 m² g⁻¹, more preferably 100 to 250 m²g⁻¹.

The oxidation catalyst for use in the present invention optionallyfurther comprises a second support material. Typically, the alkalineearth metal component is disposed or supported on the support materialcomprising the modified alumina and/or a second support material. Whenthe oxidation catalyst for use in the present invention comprises aplurality of layers, then the second support material and the supportmaterial comprising the modified alumina are preferably in differentlayers.

In general, the alkaline earth metal component is disposed or supportedon at least one support material that comprises, or consists essentiallyof, a modified alumina incorporating a heteroatom component. Typically,the catalyst for use in the invention comprises a single supportmaterial, which support material comprises, or consists essentially of,the modified alumina incorporating a heteroatom component.

If a second support material is present, especially when the secondsupport material is in the same layer as the first support material,then it is preferred that the alkaline earth metal component issubstantially disposed or supported on the support material comprisingthe modified alumina (the term “substantially” in this context refers toat least 90%, preferably at least 99%, more preferably at least 99%, ofthe mass of the alkaline earth component that is present, typically inthe layer or otherwise, is disposed on the support material comprisingthe modified alumina). It is further preferred that the alkaline earthmetal component is only disposed or supported on the support materialcomprising the modified alumina. For some combinations of supportmaterials in the same layer, it may be difficult to control the preciselocation of the alkaline earth metal component because of its solubilityand the alkaline earth metal component may be disposed or support on allof the support materials.

The oxidation catalyst for use in the present invention also comprises aplatinum group metal (PGM) component selected from the group consistingof a platinum (Pt) component, a palladium (Pd) component and acombination thereof. The oxidation catalyst for use in the invention maycomprise a single platinum group metal (PGM) component, which is eithera platinum (Pt) component or a palladium (Pd) component.

Generally, it is preferred that the oxidation catalyst comprises aplatinum (Pt) component and a palladium (Pd) component (i.e. theplatinum group metal (PGM) component is a platinum (Pt) component and apalladium (Pd) component). The ratio of the total mass of the platinum(Pt) component to the total mass of the palladium (Pd) component istypically 3:1 to 1:3, preferably 2:1 to 1:2, and more preferably 1.5:1to 1:1.5, especially when, but not exclusively, the oxidation catalystcomprises a plurality of layers. However, in a particularly preferredembodiment, the total weight ratio of Pt:Pd is 4:1 to 1:1, preferably2:1 to 1:1.

Typically, the total amount of the platinum group metal (PGM) component(e.g. the total amount of the platinum (Pt) component and/or thepalladium (Pd) component) is 5 to 500 g ft⁻³. Preferably, the totalamount of the PGM component is 10 to 400 g ft⁻³, more preferably 20 to300 g ft⁻³, still more preferably, 25 to 250 g ft⁻³, and even morepreferably 35 to 200 g ft⁻³.

The total amount of the platinum group metal (PGM) component in theoxidation catalyst for use in the catalysed soot filter according to theinvention is 5 to 100 g ft⁻³, more preferably 10 to 40 g ft⁻³.

Typically, the oxidation catalyst comprises a total amount by mass ofthe platinum group metal (PGM) component of 2.0 to 8.0 g. The totalamount of PGM component that is used depends on, amongst other thingsand the size of the substrate.

In addition to the platinum group metal (PGM) component, the oxidationcatalyst for use in the invention may further comprise a noble metalcomponent. The noble metal component comprises a noble metal selectedfrom the group consisting of ruthenium (Ru), rhodium (Rh), iridium (Ir),gold (Au), silver (Ag) and a combination of two or more thereof. It ispreferred that the noble metal component comprises a noble metalselected from the group consisting of gold, silver and a combinationthereof. More preferably, the noble metal component comprises, orconsists of, gold. When the catalyst comprises gold (Au), then aplatinum group metal (PGM) component, preferably a palladium (Pd)component, is present as an alloy with gold (Au) (e.g. a palladium-goldalloy). Catalysts comprising gold (Au) can be prepared using the methoddescribed in WO 2012/120292 by the present Applicant.

The oxidation catalyst for use in the invention optionally furthercomprises a hydrocarbon adsorbent. The hydrocarbon adsorbent may beselected from a zeolite, active charcoal, porous graphite and acombination of two or more thereof. It is preferred that the hydrocarbonadsorbent is a zeolite. More preferably, the zeolite is a medium porezeolite (e.g. a zeolite having a maximum ring size of eight tetrahedralatoms) or a large pore zeolite (e.g. a zeolite having a maximum ringsize of ten tetrahedral atoms). Examples of suitable zeolites or typesof zeolite include faujasite, clinoptilolite, mordenite, silicalite,ferrierite, zeolite X, zeolite Y, ultrastable zeolite Y, AEI zeolite,ZSM-5 zeolite, ZSM-12 zeolite, ZSM-20 zeolite, ZSM-34 zeolite, CHAzeolite, SSZ-3 zeolite, SAPO-5 zeolite, offretite, a beta zeolite or acopper CHA zeolite. The zeolite is preferably ZSM-5, a beta zeolite or aY zeolite.

Typically, the zeolite has a silica to alumina molar ratio of at least25:1, preferably at least 25:1, with useful ranges of from 25:1 to1000:1, 50:1 to 500:1 as well as 25:1 to 100:1, 25:1 to 300:1, from100:1 to 250:1. Zeolites having a high molar ratio of silica to aluminashow improved hydrothermal stability.

When the catalyst comprises a hydrocarbon adsorbent, then typically thetotal amount of hydrocarbon adsorbent is 0.05 to 3.00 g in⁻³,particularly 0.10 to 2.00 g in⁻³, more particularly 0.2 to 0.8 g in⁻³.

The catalyst for use in the invention optionally further comprises anoxygen storage material. Such materials are well-known in the art. Theoxygen storage material may be selected from ceria (CeO₂) andceria-zirconia (CeO₂—ZrO₂), such as a ceria-zirconia solid solution.

Typically, at least one platinum group metal (PGM) component issupported on the support material comprising the modified aluminaincorporating a heteroatom component. Thus, a platinum (Pt) component ora palladium (Pd) component or both a platinum (Pt) component and apalladium (Pd) component is supported on the support material.

Generally, the alkaline earth metal component and at least one platinumgroup metal (PGM) component is supported on the support materialcomprising the modified alumina incorporating a heteroatom component.Thus, the oxidation catalyst for use in the invention may comprise apalladium (Pd) component and/or a platinum (Pt) component and analkaline earth metal component supported on the same support material,namely the support material comprising the modified aluminaincorporating a heteroatom component. It is preferred that a palladium(Pd) component, a platinum (Pt) component and an alkaline earth metalcomponent are supported on the support material comprising the modifiedalumina incorporating a heteroatom component.

As mentioned above, the oxidation catalyst may or may not furthercomprise a second support material. The second support material may beselected from the group consisting of alumina, silica, alumina-silica,zirconia, titania, ceria and a mixture of two or more thereof. Thesecond support material is preferably selected from the group consistingof alumina, silica, zirconia, titania and a mixture of two or morethereof, particularly alumina, silica, titania and a mixture of two ormore thereof. More preferably, the second support material comprises, orconsists of, alumina.

When the oxidation catalyst for use in the present invention comprises asecond support material, then preferably at least one platinum groupmetal (PGM) component is supported on the second support material. Aplatinum (Pt) component, a palladium (Pd) component or both a platinum(Pt) component and a palladium (Pd) component may be supported on thesecond support material.

In addition or as alternative to being supported on the support materialcomprising the modified alumina, the alkaline earth metal component maybe supported on the second support material. However, it is preferredthat the alkaline earth metal component is only supported on the supportmaterial comprising the modified alumina (i.e. the alkaline earth metalcomponent is not supported on the second support material).

If the oxidation catalyst for use in the present invention comprises anoble metal component and/or an oxygen storage material, then the noblemetal component and/or the oxygen storage material may be supported onthe support material comprising the modified alumina and/or, if present,the second support material. When the oxidation catalyst for use in thepresent invention additionally comprises an oxygen storage material anda second support material, then the oxygen storage material and thesecond support material are different (e.g. the oxygen storage materialand the second support material are not both ceria or ceria-zirconia).

Generally, the platinum group metal (PGM) component(s), the alkalineearth metal component, the support material and any optional noble metalcomponent, oxygen storage material, hydrocarbon adsorbent and/or secondstorage material are disposed or supported on the substrate.

The oxidation catalyst for use in the invention comprises a filteringsubstrate. However, the oxidation catalyst may be comprised of aplurality of substrates in series (at least one being the filteringsubstrate according to the first aspect of the invention (e.g. 2, 3 or 4substrates, at least one being the filtering substrate according to thefirst aspect of the invention)), more preferably two substrates inseries (i.e. only two substrates, at least one being the filteringsubstrate according to the first aspect of the invention). When thereare two substrates, then a first substrate may be in contact with orseparate from a second substrate. When the first substrate is separatefrom the second substrate, then preferably the distance (e.g. theperpendicular distance between faces) between an outlet end (e.g. theface at an outlet end) of the first substrate and inlet end (e.g. theface at an inlet end) of the second substrate is from 0.5 mm to 50 mm,preferably 1 mm to 40 mm, more preferably 1.5 mm to 30 mm (e.g. 1.75 mmto 25 mm), such as 2 mm to 20 mm (e.g. 3 mm to 15 mm), and still morepreferably 5 mm to 10 mm.

In general, it is preferred that the catalysed soot filter according tothe invention comprises a single filtering substrate (i.e. only onefiltering substrate).

Filtering substrates for supporting oxidation catalysts for treating theexhaust gas of a compression ignition engine are well known in the art.Generally, the substrate is a ceramic material or a metallic material.

It is preferred that the substrate is made or composed of cordierite(SiO₂—Al₂O₃—MgO), silicon carbide (SiC), Fe—Cr—Al alloy, Ni—Cr—Al alloy,or a stainless steel alloy.

Typically, the substrate is a monolith. A filtering monolith generallycomprises a plurality of inlet channels and a plurality of outletchannels, wherein the inlet channels are open at an upstream end (i.e.exhaust gas inlet side) and are plugged or sealed at a downstream end(i.e. exhaust gas outlet side), the outlet channels are plugged orsealed at an upstream end and are open at a downstream end, and whereineach inlet channel is separated from an outlet channel by a porousstructure. When the substrate is a filtering monolith, then theoxidation catalyst of the invention is typically a catalysed soot filter(CSF) or is for use as a catalysed soot filter (CSF).

When the monolith is a filtering monolith, it is preferred that thefiltering monolith is a wall-flow filter. In a wall-flow filter, eachinlet channel is alternately separated from an outlet channel by a wallof the porous structure and vice versa. It is preferred that the inletchannel and the outlet channels have a honeycomb arrangement. When thereis a honeycomb arrangement, it is preferred that the channels verticallyand laterally adjacent to an inlet channel are plugged at an upstreamend and vice versa (i.e. the channels vertically and laterally adjacentto an outlet channel are plugged at a downstream end). When viewed fromeither end, the alternately plugged and open ends of the channels takeon the appearance of a chessboard.

In principle, the filtering substrate may be of any shape or size.However, the shape and size of the filtering substrate is usuallyselected to optimise exposure of the catalytically active materials inthe catalyst to the exhaust gas. The filtering substrate may, forexample, have a tubular, fibrous or particulate form. Examples ofsuitable supporting filtering substrates include a filtering substrateof the monolithic honeycomb cordierite type, a filtering substrate ofthe monolithic honeycomb SiC type, a filtering substrate of the layeredfibre or knitted fabric type, a filtering substrate of the foam type, afiltering substrate of the crossflow type, a filtering substrate of themetal wire mesh type, a filtering substrate of the metal porous bodytype and a filtering substrate of the ceramic particle type.

Generally, the oxidation catalyst for use in the invention comprises asingle layer or a plurality of layers (e.g. 2, 3 or 4 layers) disposedon the substrate. Typically, each layer is formed by applying a washcoatcoating onto the substrate. However, preferably the filtering substrateis a filtering monolith, particularly preferably a wallflow filtersubstrate, and the oxidation catalyst for use in the invention comprisesa single layer on inlet channels thereof and a single layer on outletchannels.

The oxidation catalyst for use in the invention may comprise, or consistof, a filtering substrate and a single layer disposed on the filteringsubstrate, wherein the single layer comprises a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof; analkaline earth metal component; and the support material comprising themodified alumina incorporating a heteroatom component. The single layermay further comprise a noble metal component and/or an oxygen storagematerial and/or a hydrocarbon adsorbent and/or a second storagematerial. It is preferred that the single layer further comprises ahydrocarbon adsorbent and optionally an oxygen storage material.

When the oxidation catalyst for use in the present invention comprises,or consists of, a filtering substrate and a single layer disposed on thefiltering substrate, then preferably the single layer comprises aplatinum (Pt) component and a palladium (Pd) component (i.e. theplatinum group metal (PGM) component is a platinum (Pt) component and apalladium (Pd) component). When the single layer comprises a platinum(Pt) component and a palladium (Pd) component, then the relative amountof the platinum (Pt) component to the palladium (Pd) component can vary.

Typically, the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component is ≧35:65 (e.g. ≧7:13). It is preferred thatthe ratio by mass of the platinum (Pt) component to the palladium (Pd)component is ≧40:60 (e.g. ≧2:3), more preferably ≧42.5:57.5 (e.g.≧17:23), particularly ≧45:55 (e.g. ≧9:11), such as ≧47.5:52.5 (e.g.≧19:21), and still more preferably ≧50:50 (e.g. ≧1:1). The ratio by mass(i.e. mass ratio) of the platinum (Pt) component to the palladium (Pd)component is typically 80:20 to 35:65 (e.g. 4:1 to 7:13). It ispreferred that the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component is 75:25 to 40:60 (e.g. 3:1 to 2:3), morepreferably 70:30 to 42.5:57.5 (e.g. 7:3 to 17:23), even more preferably67.5:32.5 to 45:55 (e.g. 27:13 to 9:11), such as 65:35 to 47.5:52.5(e.g. 13:7 to 19:21), and still more preferably 60:40 to 50:50 (e.g. 3:2to 1:1). Particularly preferred is a ratio by mass of the Pt componentto the Pd component of 4:1 to 1:1, preferably 2:1 to 1:1.

It is thought that oxidation catalysts for use in the present inventionwhere the mass of the palladium (Pd) component is less than the mass ofthe platinum (Pt) component have advantageous activity. Thus, thecatalyst of the invention preferably comprises the platinum (Pt)component and the palladium (Pd) component in a ratio by mass of 65:35to 52.5:47.5 (e.g. 13:7 to 21:19), more preferably 60:40 to 55:45 (e.g.3:2 to 11:9).

Typically, the ratio by mass (i.e. mass ratio) of the alkaline earthmetal component to the platinum group metal (PGM) component is 0.25:1 to20:1. It is preferred that the mass ratio of the alkaline earth metalcomponent to the platinum group metal (PGM) component is 0.5:1 to 17:1,more preferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1.

Alternatively, the oxidation catalyst for use in the present inventioncan comprise a plurality of layers, such as 2, 3 or 4 layers.

When there is a plurality of layers, then the oxidation catalyst maycomprise a plurality of substrates (at least one being the filteringsubstrate according to the first aspect of the invention), preferablytwo substrates (at least one being the filtering substrate according tothe first aspect of the invention). When there is a plurality ofsubstrates (at least one being the filtering substrate according to thefirst aspect of the invention (e.g. two substrates)), then in oneembodiment a first layer is disposed on a first substrate and a secondlayer is disposed on a second substrate. Thus, any reference below tothe first layer being disposed on the substrate may refer to the firstlayer being disposed on the first substrate. Similarly, any referencebelow to the second layer being disposed on the second substrate mayrefer to the second layer being disposed on the second substrate.

When there is a plurality of substrates (at least one being thefiltering substrate according to the first aspect of the invention),then the first substrate may be upstream of the second substrate.Alternatively, the second substrate may be upstream of the firstsubstrate.

In general, it is preferred that the catalysed soot filter according tothe invention comprises a single filtering substrate, particularly whenthe oxidation catalyst comprises a plurality of layers.

When there is a plurality of layers, then generally a first layer isdisposed on the filtering substrate (e.g. the first layer is preferablydisposed directly on the substrate, such that the first layer is incontact with a surface of the substrate). The first layer may bedisposed on a third layer or a fourth layer. It is preferable that thefirst layer is disposed directly on the filtering substrate.

A second layer may be disposed on the filtering substrate (e.g. to forma zone as described below, which is separate from, or partly overlapswith, the first layer) or the second layer may be disposed on the firstlayer.

When the second layer is disposed on the first layer, it may completelyor partly overlap (i.e. cover) the first layer. If the catalystcomprises a third layer, then the third layer may be disposed on thesecond layer and/or the first layer, preferably the third layer isdisposed on the first layer. If the catalyst comprises a fourth layer,then the fourth layer may be disposed on the third layer and/or thesecond layer.

When the second layer is disposed on the filtering substrate (e.g. toform a zone), then the second layer may be disposed directly on thefiltering substrate (i.e. the second layer is in contact with a surfaceof the filtering substrate) or it may be disposed on a third layer or afourth layer.

The first layer may be a zone (e.g. a first zone) and/or the secondlayer may be a zone (e.g. a second zone). For the avoidance of doubt,features described herein relating to the “first layer” and “secondlayer”, especially the composition of the “first layer” and the “secondlayer”, also relate to the “first zone” and “second zone” respectively.

The first layer may be a first zone and the second layer may be a secondzone, such as when the first zone and second zone are side-by-side onthe same filtering substrate or the first zone is disposed on a firstsubstrate and a second zone is disposed on a second substrate (i.e. thefirst substrate and the second substrate are different) and the firstsubstrate and second substrate are side-by-side. Preferably, the firstzone and second zone are disposed on the same filtering substrate.

The first zone may be upstream of the second zone. When the first zoneis upstream of the second zone, inlet exhaust gas will contact the firstzone before the second zone. Alternatively, the second zone may beupstream of the first zone. Similarly, when the second zone is upstreamof the first zone, inlet exhaust gas will contact the second zone beforethe first zone.

When the first zone and the second zone are disposed on the samefiltering substrate, then the first zone may abut the second zone or thefirst zone may be separate from the second zone. If the first zone abutsthe second zone, then preferably the first zone is in contact with thesecond zone. When the first zone is separate from the second zone, thentypically there is a gap or space between the first zone and the secondzone.

Typically, the first zone has a length of 10 to 80% of the length of thefiltering substrate (e.g. 10 to 45%), preferably 15 to 75% of the lengthof the filtering substrate (e.g. 15 to 40%), more preferably 20 to 60%(e.g. 25 to 45%) of the length of the filtering substrate, still morepreferably 25 to 50%.

The second zone typically has a length of 10 to 80% of the length of thefiltering substrate (e.g. 10 to 45%), preferably 15 to 75% of the lengthof the filtering substrate (e.g. 15 to 40%), more preferably 20 to 60%(e.g. 25 to 45%) of the length of the filtering substrate, still morepreferably 25 to 50%.

An oxidation catalyst for use in the present invention comprises twolayers (e.g. only two layers), wherein a first layer is disposed on thesubstrate and a second layer is disposed on the first layer.

Typically, the second layer completely or partly overlaps the firstlayer.

The first layer and the second layer may have different lengths, or thefirst layer and the second layer may have about the same length.Generally, the length of the first layer and the length of the secondlayer is each substantially uniform.

The first layer typically extends for substantially an entire length ofthe channels in the substrate, particularly when the filtering substrateis a monolith.

In an oxidation catalyst for use in the present invention comprising aplurality of layers, the second layer may be arranged in a zone ofsubstantially uniform length at a downstream end of the filteringsubstrate. It is preferred that the zone at the downstream end is nearerto the outlet end of the filtering substrate than to the inlet end.Methods of making differential length layered coatings are known in theart (see for example WO 99/47260 by the present Applicant).

When the oxidation catalyst for use in the present invention comprises aplurality of layers, then the platinum group metal (PGM) component, thealkaline earth metal component and the support material comprising themodified alumina can be distributed amongst the layers in a variety ofways.

In general, the first layer (or first zone) comprises a platinum groupmetal (PGM) component selected from the group consisting of a platinum(Pt) component, a palladium (Pd) component and a combination thereof,and the second layer (or second zone) comprises a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof. It ispreferred that the first layer/zone is different (e.g. in composition)to the second layer/zone. For example, the first and second layers/zonesmay comprise different platinum group metal (PGM) components and/or thefirst and second layers/zones may comprise a different total amount ofthe platinum group metal (PGM) component.

In a first embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pd component and acombination of (i.e. both) a Pd component and a Pt component, and thesecond layer (or second zone) comprises a PGM component consisting of aPt component. This means that the first layer/zone comprises a Pdcomponent and optionally a Pt component as the only PGM component andthe second layer/zone comprises a Pt component as the only PGMcomponent. Preferably, the first layer/zone comprises a PGM componentconsisting of a combination of (i.e. both) a Pd component and a Ptcomponent. Thus, it is preferred that the first layer/zone comprisesboth a Pt component and a Pd component as the only PGM component, andthe second layer/zone comprises a Pt component as the only PGMcomponent.

Typically, in the first embodiment, the first layer (or first zone)further comprises the alkaline earth metal component and the supportmaterial comprising a modified alumina incorporating a heteroatomcomponent, and/or the second layer (or second zone) further comprisesthe alkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. It is preferredthat the first layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

When the first layer/zone comprises a Pd component as the only PGMcomponent, then the first layer/zone may comprise a second supportmaterial. Preferably the second support material is ceria,ceria-zirconia, alumina or silica-alumina. The second support materialmay be ceria. The second support material may be ceria-zirconia. Thesecond support material may be alumina. The second support material maybe silica-alumina. More preferably, the first layer/zone comprises a PGMcomponent selected from the group consisting of a Pd component, and asecond support material, wherein the second support material is ceria.

In a second embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and acombination of (i.e. both) a Pd component and a Pt component, and thesecond layer (or second zone) comprises a PGM component consisting of aPd component. This means that the first layer/zone comprises a Ptcomponent and optionally a Pd component as the only PGM component andthe second layer/zone comprises a Pd component as the only PGMcomponent. Preferably, the first layer/zone comprises a PGM componentconsisting of a combination of (i.e. both) a Pd component and a Ptcomponent. Thus, it is preferred that the first layer/zone comprisesboth a Pt component and a Pd component as the only PGM component, andthe second layer/zone comprises a Pd component as the only PGMcomponent. Typically, the amount of the Pt component in the firstlayer/zone is greater than the amount of the Pd component in the firstlayer/zone (the amount being measured in g ft⁻³ or as a molar amount).

In the second embodiment, the first layer (or first zone) may furthercomprise the alkaline earth metal component and the support materialcomprising a modified alumina incorporating a heteroatom component,and/or the second layer (or second zone) may further comprise thealkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. It is preferredthat the first layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

In the second embodiment, the second layer/zone typically comprises asecond support material. Preferably the second support material isceria, ceria-zirconia, alumina or silica-alumina. The second supportmaterial may be ceria. The second support material may beceria-zirconia. The second support material may be alumina. The secondsupport material may be silica-alumina.

In a third embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and a Pdcomponent, and the second layer (or second zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pd component anda Pt component. This means that the first layer/zone comprises a Ptcomponent or a Pd component as the only PGM component, and the secondlayer/zone comprises a Pt component and a Pd component as the only PGMcomponent. Preferably, the first layer/zone comprises a PGM componentconsisting of a Pt component. Thus, it is preferred that the firstlayer/zone comprises a Pt component as the only PGM component, and thesecond layer/zone comprises a Pt component and a Pd component as theonly PGM component.

In the third embodiment, when the first layer/zone comprises a Ptcomponent as the only PGM component, then typically the ratio by mass ofthe Pt component in the second layer/zone to the Pd component in thesecond layer/zone is ≦2:1, preferably <2:1. When the first layer/zonecomprises a Pd component as the only PGM component, then typically theamount of the Pd component in the second layer/zone is less than theamount of the Pt component in the second layer/zone (the amount beingmeasured in g ft⁻³ or is a molar amount).

Typically, in the third embodiment, the first layer (or first zone)further comprises the alkaline earth metal component and the supportmaterial comprising a modified alumina incorporating a heteroatomcomponent, and/or the second layer (or second zone) further comprisesthe alkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. When the firstlayer/zone comprises a Pt component as the only PGM component, then itis preferred that the first layer/zone further comprises the alkalineearth metal component and the support material comprising a modifiedalumina incorporating a heteroatom component. When the first layer/zonecomprises a Pd component as the only PGM component, then it is preferredthat the second layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

In the third embodiment, when the first layer/zone comprises a Pdcomponent as the only PGM component, then the first layer/zone maycomprise a second support material. Preferably the second supportmaterial is ceria, ceria-zirconia, alumina or silica-alumina. The secondsupport material may be ceria. The second support material may beceria-zirconia. The second support material may be alumina. The secondsupport material may be silica-alumina.

In a fourth embodiment, the first layer (or first zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pt component anda Pd component, and the second layer (or second zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pd component anda Pt component. This means that first layer/zone comprises a Ptcomponent and a Pd component as the only PGM component, and the secondlayer/zone comprises a Pt component or a Pd component as the only PGMcomponent. In the fourth embodiment, the first layer/zone and secondlayer/zone typically comprise a different ratio by mass of the Ptcomponent to the Pd component. Thus, the ratio by mass of the Ptcomponent to the Pd component in the first layer/zone is different tothe ratio by mass of the Pt component to the Pd component in thesecond/zone layer.

In the fourth embodiment, when the amount of the Pd component in thefirst layer/zone is less than the amount of the Pt component in thefirst layer/zone (the amount being measured in gft⁻³ or is a molaramount), then preferably the amount of the Pd component in the secondlayer/zone is greater than the amount of the Pt component in the secondlayer/zone. Alternatively, when the amount of the Pd component in thefirst layer/zone is greater than the amount of the Pt component in thefirst layer/zone (the amount being measured in g ft⁻³ or is a molaramount), then preferably the amount of the Pd component in the secondlayer/zone is less than the amount of the Pt component in the secondlayer/zone.

Generally, the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component, particularly in first layer/zone of the firstor second embodiments, the second layer/zone of the third embodiment, orthe first layer/zone and/or second layer/zone of the fourth embodiment,preferably the second layer/zone of the fourth embodiment, is ≧35:65(e.g. ≧7:13). It is preferred that the ratio by mass of the platinum(Pt) component to the palladium (Pd) component is ≧40:60 (e.g. ≧2:3),more preferably ≧42.5:57.5 (e.g. ≧17:23), particularly ≧45:55 (e.g.≧9:11), such as ≧47.5:52.5 (e.g. ≧19:21), and still more preferably≧50:50 (e.g. ≧1:1).

It is preferred that the ratio by mass of the platinum (Pt) component tothe palladium (Pd) component, particularly in first layer/zone of thefirst or second embodiments, the second layer/zone of the thirdembodiment, or the first layer/zone and/or second layer/zone of thefourth embodiment, preferably the second layer/zone of the fourthembodiment, is 80:20 to 35:65 (e.g. 4:1 to 7:13), particularly 75:25 to40:60 (e.g. 3:1 to 2:3), more preferably 70:30 to 42.5:57.5 (e.g. 7:3 to17:23), even more preferably 67.5:32.5 to 45:55 (e.g. 27:13 to 9:11),such as 65:35 to 47.5:52.5 (e.g. 13:7 to 19:21), and still morepreferably 60:40 to 50:50 (e.g. 3:2 to 1:1). For the second layer of thethird embodiment, it is particularly preferable that the ratio by massof the platinum (Pt) component to the palladium (Pd) component is 2:1 to7:13, particularly 13:7 to 2:3, more preferably 60:40 to 50:50 (e.g. 3:2to 1:1)

It is thought that oxidation catalysts for use in the present inventionwhere the mass of the palladium (Pd) component is less than the mass ofthe platinum (Pt) component have advantageous activity, especially whenboth a platinum (Pt) component, a palladium (Pd) component and analkaline earth metal component are present in the same layer/zone. Thus,in the first layer/zone of the first embodiment, the first layer/zone ofthe second embodiment, the second layer/zone of the third embodiment, orthe first layer/zone and/or second layer/zone of the fourth embodiment,preferably the second layer/zone of the fourth embodiment, the oxidationcatalyst of the invention preferably comprises the platinum (Pt)component and the palladium (Pd) component in a ratio by mass of 65:35to 52.5:47.5 (e.g. 13:7 to 21:19), more preferably 60:40 to 55:45 (e.g.3:2 to 11:9).

In a fifth embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and a Pdcomponent, and the second layer (or second zone) comprises a PGMcomponent selected from the group consisting of a Pd component and a Ptcomponent, and wherein the first and second layer/zone each comprise thesame PGM component. This means that the first layer/zone and the secondlayer/zone each comprise a Pt component or a Pd component as the onlyPGM component. Typically, the total amount of PGM component in the firstlayer/zone is different to the total amount of PGM component in thesecond layer/zone.

When both the first layer/zone and the second layer/zone each comprise aPd component as the only PGM component, then preferably the firstlayer/zone comprises a second support material and/or the secondlayer/zone comprises a second support material. It is preferred that thesecond support material is ceria, ceria-zirconia, alumina orsilica-alumina. The second support material may be ceria. The secondsupport material may be ceria-zirconia. The second support material maybe alumina. The second support material may be silica-alumina.

In the first to fifth embodiments, the first layer/zone may comprise analkaline earth metal component and/or the second layer/zone may comprisean alkaline earth metal component. When the first layer/zone comprisesthe alkaline earth metal component, the second layer/zone may notcomprise an alkaline earth metal component. Alternatively, when thesecond layer/zone comprises the alkaline earth metal component, thefirst layer/zone may not comprise an alkaline earth metal component.

In the first to fifth embodiments, the first layer/zone may comprise thesupport material comprising the modified alumina, and/or the secondlayer/zone may comprise the support material comprising the modifiedalumina. Typically, it is preferred that a layer or zone comprising aplatinum (Pt) component also comprises the support material comprisingthe modified alumina.

In the first to fifth embodiments, the first layer/zone may comprise asecond support material and/or the second layer/zone may comprise asecond support material. The first layer/zone and the second layer/zonemay comprise different support materials. It is preferred that thesecond support material and the support material comprising the modifiedalumina are in different layers/zones.

In general, the alkaline earth metal component and the support materialcomprising the modified alumina are present in at least one of the samelayers/zones.

When the first layer/zone comprises the alkaline earth metal component,then typically the ratio of the mass of the alkaline earth metalcomponent to the mass of the platinum group metal (PGM) component in thefirst layer is 0.25:1 to 20:1, preferably 0.5:1 to 17:1, more preferably1:1 to 15:1, particularly 1.5:1 to 10:1, still more preferably 2:1 to7.5:1, and even more preferably 2.5:1 to 5:1.

When the second layer/zone comprises the alkaline earth metal component,then typically the ratio of the mass of the alkaline earth metalcomponent to the mass of the platinum group metal (PGM) component in thesecond layer is 0.25:1 to 20:1, preferably 0.5:1 to 17:1, morepreferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1.

In the first to fifth embodiments, the first layer/zone may optionallyfurther comprise a noble metal component and/or an oxygen storagematerial and/or a hydrocarbon adsorbent. Preferably, the firstlayer/zone further comprises a hydrocarbon adsorbent.

In the first to fifth embodiments, the second layer/zone may optionallyfurther comprise a noble metal component and/or an oxygen storagematerial and/or a hydrocarbon adsorbent. Preferably, the secondlayer/zone further comprises a hydrocarbon adsorbent.

In one arrangement of the first embodiment, the first layer/zonetypically comprises a Pd component, a Pt component, an alkaline earthmetal component and the support material comprising the modifiedalumina; and the second layer/zone comprises a Pt component and either asecond support material or a support material comprising the modifiedalumina, and optionally an alkaline earth metal component. When thesecond layer/zone comprises a second support material, then preferablythe second support material is alumina.

In one arrangement of the fourth embodiment, the first layer/zonecomprises a Pt component, a Pd component, an alkaline earth metalcomponent and the support material comprising the modified alumina; andthe second layer/zone comprises a Pt component, a Pd component andeither a second support material or a support material comprising themodified alumina, and optionally an alkaline earth metal component. Itis preferred that the ratio by mass of the Pt component in the secondlayer/zone to the Pd component in the second layer is ≦10:1 (e.g. 10:1to 1:2), more preferably ≦15:2 (e.g. 7.5:1 to 1:1.5), and still morepreferably ≦5:1 (e.g. 5:1 to 1.5:1). When the second layer/zonecomprises a second support material, then preferably the second supportmaterial is alumina.

When the first layer is a first zone and the second layer is a secondzone, then (a) in the first and third embodiments it is preferred thatthe first layer/zone is upstream of the second layer/zone, (b) in thesecond embodiment it is preferred that the second layer/zone is upstreamof the first layer/zone, and (c) in the fifth embodiment it is preferredthat the layer/zone comprising the second support material is upstreamof the layer/zone comprising the support material comprising themodified alumina.

In embodiments where there is a second support material, especially whenthe second support material is either ceria or ceria-zirconia, then itmay be advantageous to arrange the layer or zone comprising the secondsupport material to contact the exhaust gas after the other layer orzone. Thus, when there is a second support material, especially when thesecond support material is ceria or ceria-zirconia, it is preferred that(a) in the first and third embodiments it is preferred that the firstlayer/zone is downstream of the second layer/zone, (b) in the secondembodiment it is preferred that the second layer/zone is downstream ofthe first layer/zone, and (c) in the fifth embodiment it is preferredthat the layer/zone comprising the second support material is downstreamof the layer/zone comprising the support material comprising themodified alumina.

In general, the oxidation catalyst of the invention may or may notcomprise rhodium. It is preferred that the oxidation catalyst does notcomprise ruthenium, rhodium, and iridium.

Another general feature of the oxidation catalyst for use in theinvention is that when any cerium or ceria is present, then typicallyonly the heteroatom component of the support material comprises thecerium or ceria. It is further preferred that the oxidation catalyst ofthe invention does not comprise ceria, particularly as a supportmaterial or as an oxygen storage material.

A further general feature of the oxidation catalyst of the invention isthat when an alkali metal, particularly sodium or potassium, andespecially potassium, is present, then preferably only the hydrocarbonadsorbent comprises the alkali metal, especially when the hydrocarbonadsorbent is a zeolite. It is further preferred that the oxidationcatalyst for use in the invention does not comprise an alkali metal,particularly sodium or potassium.

Another general feature of the invention is that the oxidation catalystfor use in the invention does not comprise a NO_(x) adsorbercomposition. Thus, it is preferred that the oxidation catalyst of theinvention is not a NO_(x) adsorber catalyst (also known as a NO_(x)trap) or is not for use as a NO_(x) adsorber catalyst.

The first aspect of the invention relates to a catalysed soot filtercomprising an oxidation catalyst as defined above. The second aspect ofthe invention relates to an exhaust system for a compression ignitionengine, such as a diesel engine, which system comprises the catalysedsoot filter as defined above. The fifth aspect of the invention relatesto the use of the catalysed soot filter. The advantageous activity ofthe oxidation catalyst for use in the invention, particularly its low CO“light off” temperature, render it particularly suited for use incombination with certain other emissions control devices.

The second aspect of the invention relates to an exhaust systemcomprising a catalysed soot filter according to the invention.Typically, the exhaust system according to the invention may furthercomprise, or the catalysed soot filter is for use in combination with,at least one emissions control device, preferably disposed on a separatesubstrate monolith located either upstream or downstream of the CSFaccording to the invention. The emissions control device may be selectedfrom a diesel particulate filter (DPF) (i.e. an uncatalysed filter,which may be a bare filter or a filter washcoated with a non-catalyticwashcoat, e.g. alumina or as described hereinbelow), a NO_(x) adsorbercatalyst (NAC), a lean NO_(x) catalyst (LNC), a selective catalyticreduction (SCR) catalyst, a diesel oxidation catalyst (DOC), a catalysedsoot filter (CSF), a filter catalysed with a selective catalyticreduction catalyst, an ammonia slip catalyst (ASC) and combinations oftwo or more thereof. Emissions control devices represented by the termsdiesel particulate filters (DPFs), NO_(x) adsorber catalysts (NACs),lean NO_(x) catalysts (LNCs), selective catalytic reduction (SCR)catalysts, diesel oxidation catalyst (DOCs), catalysed soot filters(CSFs) and filters catalysed with a selective catalytic reductioncatalyst are all well known in the art.

A highly preferred exhaust system according to the present inventioncomprises a diesel oxidation catalyst (DOC) disposed on a separateflow-through substrate monolith, which is disposed upstream of thecatalysed soot filter. The diesel oxidation catalyst formulation can bean oxidation catalyst described herein, i.e. a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof; analkaline earth metal component; a support material comprising a modifiedalumina incorporating a heteroatom component. However, it is preferredto use a DOC composition which is active for NO oxidation therebyproducing NO₂ for passive combustion of particulate matter trapped onthe downstream CSF device, according to the well-known CRT® technologyeffect, i.e. NO₂+C→CO+NO. Therefore in order to promote NO oxidation,low alkaline earth loadings are preferred, or in one embodiment, theupstream DOC contains substantially no alkaline earth metal. Such“no-alkaline earth metal” DOC can otherwise have a composition asdescribed for the oxidation catalyst herein.

A flow-through monolith typically comprises a honeycomb monolith (e.g. ametal or ceramic honeycomb monolith) having a plurality of channelsextending therethrough, which channels are open at both ends. When thesubstrate is a flow-through monolith, then the oxidation catalyst of theinvention is typically a diesel oxidation catalyst (DOC) or is for useas a diesel oxidation catalyst (DOC).

Examples of emissions control devices for use with the catalysed sootfilter according to the invention or for inclusion in the exhaust systemof the invention are provided below.

The catalyst formulation of the diesel particulate filter may be aNO_(x) adsorber composition. When the catalyst formulation is a NO_(x)adsorber composition, the emissions control device is an example of aNO_(x) adsorber catalyst (NAC). Emissions control devices where thecatalyst formulation is a NO_(x) adsorber composition have beendescribed (see, for example, EP 0766993). NO_(x) adsorber compositionsare well known in the art (see, for example, EP 0766993 and U.S. Pat.No. 5,473,887). NO_(x) adsorber compositions are designed to adsorbNO_(x) from lean exhaust gas (lambda >1) and to desorb the NO_(x) whenthe oxygen concentration in the exhaust gas is decreased. DesorbedNO_(x) may then be reduced to N₂ with a suitable reductant (e.g. enginefuel) and promoted by a catalyst component, such as rhodium, of theNO_(x) adsorber composition itself or located downstream of the NO_(x)adsorber composition.

Modern NO_(x) absorber catalysts coated on honeycomb flow-throughmonolith substrates are typically arranged in layered arrangements.However, multiple layers applied on a filter substrate can createbackpressure problems. It is highly preferable, therefore, if the NO_(x)absorber catalyst for use in the present invention is a “single layer”NO_(x) absorber catalyst. Particularly preferred “single layer” NO_(x)absorber catalysts comprise a first component of rhodium supported on aceria-zirconia mixed oxide or an optionally stabilised alumina (e.g.stabilised with silica or lanthana or another rare earth element) incombination with second components which support platinum and/orpalladium. The second components comprise platinum and/or palladiumsupported on an alumina-based high surface area support and aparticulate “bulk” ceria (CeO₂) component, i.e. not a soluble ceriasupported on a particulate support, but “bulk” ceria capable ofsupporting the Pt and/or Pd as such. The particulate ceria comprises aNO_(x) absorber component and supports an alkaline earth metal and/or analkali metal, preferably barium, in addition to the platinum and/orpalladium. The alumina-based high surface area support can be magnesiumaluminate e.g. MgAl₂O₄, for example.

The preferred “single layer” NAC composition comprises a mixture of therhodium and platinum and/or palladium support components. Thesecomponents can be prepared separately, i.e. pre-formed prior tocombining them in a mixture, or rhodium, platinum and palladium saltsand the supports and other components can be combined and the rhodium,platinum and palladium components hydrolysed preferentially to depositonto the desired support.

Generally, a NO_(x) adsorber composition comprises an alkali metalcomponent, an alkaline earth metal component or a rare earth metalcomponent or a combination of two or more components thereof, whereinthe rare earth metal component comprises lanthanum or yttrium. It ispreferred that the alkali metal component comprises potassium or sodium,more preferably potassium. It is preferred that the alkaline earth metalcomponent comprises barium or strontium, more preferably barium.

The NO_(x) adsorber composition may further comprise a support materialand/or a catalytic metal component. The support material may be selectedfrom alumina, ceria, titania, zirconia and mixtures thereof. Thecatalytic metal component may comprise a metal selected from platinum(Pt), palladium (Pd), rhodium (Rh) and combinations of two or morethereof.

Lean NO_(x) catalysts (LNCs) are well known in the art. Preferred leanNO_(x) catalysts (LNC) comprises either (a) platinum (Pt) supported onalumina or (b) a copper exchanged zeolite, particularly copper exchangedZSM-5.

SCR catalysts are also well known in the art. Preferably, a substratemonolith comprising the SCR catalyst or filter comprising the SCRcatalyst is disposed downstream of the CSF according to the invention(or, in the preferred embodiment, the combination of DOC+CSF). When theexhaust system of the invention comprises an SCR catalyst, then theexhaust system may further comprise an injector for injecting anitrogenous reductant, such as ammonia or an ammonia precursor such asurea or ammonium formate, preferably urea, into exhaust gas downstreamof the catalyst for oxidising carbon monoxide (CO) and hydrocarbons(HCs) and upstream of the SCR catalyst. Such injector is fluidly linkedto a source of such nitrogenous reductant precursor, e.g. a tankthereof, and valve-controlled dosing of the precursor into the exhauststream is regulated by suitably programmed engine management means andclosed loop or open loop feedback provided by sensors monitoringrelevant exhaust gas composition Ammonia can also be generated byheating ammonium carbamate (a solid) and the ammonia generated can beinjected into the exhaust gas.

Alternatively or in addition to the injector, ammonia can be generatedin situ e.g. during rich regeneration of a NAC disposed upstream of thefilter or by contacting a DOC disposed upstream of the filter withengine-derived rich exhaust gas (see the alternatives to reactions (4)and (5) hereinabove).

SCR catalysts for use in the present invention promote the reactionsselectively 4NH₃+4NO+O₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO);4NH₃+2NO+2NO₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO_(x); and 8NH₃+6NO₂→7N₂+12H₂O(i.e. 4:3 NH₃:NO_(x)) in preference to undesirable, non-selectiveside-reactions such as 2NH₃+2NO₂=N₂O+3H₂O+N₂.

The SCR catalyst may comprise a metal selected from the group consistingof at least one of Cu, Hf, La, Au, In, V, lanthanides and Group VIIItransition metals, such as Fe, which metal is supported on a refractoryoxide or molecular sieve. Particularly preferred metals are Ce, Fe andCu and combinations of any two or more thereof.

The refractory oxide may be selected from the group consisting of Al₂O₃,TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxides containing two or more thereof.The non-zeolite catalyst can also include tungsten oxide, e.g.V₂O₅/WO₃/TiO₂, WO_(x)/CeZrO₂, WO_(x)/ZrO₂ or Fe/WO_(x)/ZrO₂.

It is particularly preferred when an SCR catalyst or washcoat thereofcomprises at least one molecular sieve, such as an aluminosilicatezeolite or a SAPO. The at least one molecular sieve can be a small, amedium or a large pore molecular sieve, for example. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCRcatalysts—see for example WO 2008/132452.

Preferred molecular sieves with application as SCR catalysts in thepresent invention are synthetic aluminosilicate zeolite molecular sievesselected from the group consisting of AEI, ZSM-5, ZSM-20, ERI includingZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEV includingNu-3, MCM-22 and EU-1, preferably AEI or CHA, and having asilica-to-alumina ratio of about 10 to about 50, such as about 15 toabout 40.

At its most basic, an ammonia slip catalyst (ASC) can be an oxidationcatalyst for oxidising ammonia which slips past an upstream SCR or SCRcatalysed filter unreacted. The desired reaction (simplified) can berepresented by 4NO+4NH₃+O₂→4N₂+6H₂O. Ammonia is a strong smellingcompound and potential irritant to animal mucosal surfaces, e.g. eyesand respiratory pathways, and so its emission to atmosphere should belimited so far as possible. Possible ammonia slip catalysts includerelatively low loaded platinum group metals, preferably including Pte.g. 1-15 g/ft³, on a suitable relatively high surface area oxidesupport, e.g. alumina coated on a suitable substrate monolith.

In a particularly preferred arrangement, however, the platinum groupmetal and the support material (e.g. comprising a modified aluminaincorporating a heteroatom component) is disposed on a substrate (i.e.substrate monolith) in a first layer below an upper, second layeroverlying the first layer. The second layer is a SCR catalyst, selectedfrom any of those mentioned hereinabove, particularly molecular sievescontaining transition metals, such as Cu or Fe. A particularly preferredASC in the layered arrangement comprises CuCHA in the second or upperlayer.

In a first exhaust system embodiment, the exhaust system comprises thecatalysed soot filter according to the invention with a substratemonolith disposed upstream comprising a DOC. Such an arrangement may becalled a DOC/CSF. The oxidation catalyst is typically followed by (e.g.is upstream of) the catalysed soot filter (CSF). Thus, for example, anoutlet of the oxidation catalyst is connected to an inlet of thecatalysed soot filter.

The first exhaust system embodiment may further comprise a lean NO_(x)catalyst (LNC). Thus, the embodiment further relates to the use of theoxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a NO_(x) adsorber catalyst (NAC) andthe catalysed soot filter (CSF) according to the invention. Typically, aDOC is followed by (e.g. is upstream of) the NO_(x) adsorber catalyst(NAC), and the NO adsorber catalyst (NAC) is followed by (e.g. isupstream of) the catalysed soot filter (CSF) according to the invention.Generally, the DOC, the NO_(x) adsorber catalyst (NAC) and the catalysedsoot filter (CSF) are connected in series. Thus, for example, an outletof the DOC is connected to an inlet of the NO_(x) adsorber catalyst(NAC), and an outlet of the NO_(x) adsorber catalyst is connected to aninlet of the catalysed soot filter (CSF) according to the invention.Such an arrangement may be termed a DOC/NAC/CSF.

In a second exhaust system embodiment, the exhaust system comprises adiesel oxidation catalyst and the catalysed soot filter (CSF) of theinvention. This arrangement may also be called a DOC/CSF arrangement.The embodiment further relates to the use of the catalysed soot filteraccording to the invention for treating an exhaust gas from acompression ignition engine in combination with a diesel oxidationcatalyst (DOC). Typically, the diesel oxidation catalyst (DOC) isfollowed by (e.g. is upstream of) the catalysed soot filter according tothe invention. Thus, an outlet of the diesel oxidation catalyst isconnected to an inlet of the catalysed soot filter of the invention.

A third exhaust system embodiment relates to an exhaust systemcomprising a DOC, a catalysed soot filter (CSF) according to theinvention and a selective catalytic reduction (SCR) catalyst. Such anarrangement may be called a DOC/CSF/SCR and can be for use in a heavyduty diesel vehicle or a light duty diesel vehicle, preferably anexhaust system for a light-duty diesel vehicle. This embodiment alsorelates to the use of the catalysed soot filter of the invention fortreating an exhaust gas from a compression ignition engine incombination with a diesel oxidation catalyst (DOC) and a selectivecatalytic reduction (SCR) catalyst. The diesel oxidation catalyst (DOC)is typically followed by (e.g. is upstream of) the catalysed soot filter(CSF). The catalysed soot filter is typically followed by (e.g. isupstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the catalysedsoot filter (CSF) and the selective catalytic reduction (SCR) catalyst.Thus, the catalysed soot filter (CSF) may be followed by (e.g. isupstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction (SCR) catalyst.

A fourth exhaust system embodiment relates to an exhaust systemcomprising a NO_(x) adsorber catalyst (NAC), the catalysed soot filterof the invention and a selective catalytic reduction (SCR) catalyst.This is also a NAC/CSF/SCR arrangement. A further aspect of thisembodiment relates to the use of the catalysed soot filter for treatingan exhaust gas from a compression ignition engine in combination with aNO_(x) adsorber catalyst (NAC) and a selective catalytic reduction (SCR)catalyst. The NO_(x) adsorber catalyst (NAC) is typically followed by(e.g. is upstream of) the catalysed soot filter of the invention. Thecatalysed soot filter of the invention is typically followed by (e.g. isupstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, theoxidation catalyst may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. Alternatively, or additionally to thenitrogenous reductant injector, ammonia can be generated in situ e.g.during rich regeneration of a NAC disposed upstream of the filter.

In a fifth exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a DOC, a selectivecatalytic reduction (SCR) catalyst and either a catalysed soot filter(CSF) or a diesel particulate filter (DPF). The arrangement is either aDOC/SCR/CSF or a DOC/SCR/DPF. This embodiment also relates to the use ofthe oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a selective catalytic reduction(SCR) catalyst and either a catalysed soot filter (CSF) or a dieselparticulate filter (DPF), preferably wherein the oxidation catalyst is,or is for use as, a diesel oxidation catalyst.

In the fifth exhaust system embodiment, the catalysed soot filter of theinvention is typically followed by (e.g. is upstream of) the selectivecatalytic reduction (SCR) catalyst.

A nitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, thecatalysed soot filter of the invention may be followed by (e.g. isupstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction (SCR) catalyst. The selective catalyticreduction (SCR) catalyst are followed by (e.g. are upstream of) thecatalysed soot filter (CSF) of the invention.

A sixth exhaust system embodiment comprises the catalysed soot filter ofthe invention and a filter comprising a selective catalytic reductioncatalyst. Such an arrangement may be called a CSF/SCR Filter. Thisembodiment also relates to the use of the catalysed soot filter fortreating an exhaust gas from a compression ignition engine incombination with a selective catalytic reduction filter catalyst. Thecatalysed soot filter of the invention is typically followed by (e.g. isupstream of) the selective catalytic reduction filter catalyst. Anitrogenous reductant injector may be arranged between the catalysedsoot filter of the invention and the selective catalytic reductionfilter catalyst. Thus, the catalysed soot filter of the invention may befollowed by (e.g. is upstream of) a nitrogenous reductant injector, andthe nitrogenous reductant injector may be followed by (e.g. is upstreamof) the selective catalytic reduction filter catalyst.

In a seventh exhaust system embodiment, the exhaust system comprises aNO_(x) adsorber catalyst (NAC) and the catalysed soot filter (CSF) ofthe invention. This arrangement may also be called a NAC/CSFarrangement. The embodiment further relates to the use of the catalysedsoot filter for treating an exhaust gas from a compression ignitionengine in combination with a NO_(x) adsorber catalyst (NAC). Typically,the catalysed soot filter (CSF) is downstream of the NO_(x) adsorbercatalyst (NAC). Thus, an outlet of the NO_(x) adsorber catalyst (NAC) isconnected to an inlet of the catalysed soot filter of the invention.

The seventh exhaust system embodiment may further comprise a selectivecatalytic reduction (SCR) catalyst. Thus, the embodiment further relatesto the use of the catalysed soot filter for treating an exhaust gas froma compression ignition engine in combination with a NO_(x) adsorbercatalyst (NAC) and a selective catalytic reduction (SCR) catalyst.Typically the NO_(x) adsorber catalyst (NAC) is followed by (e.g. isupstream of) the catalysed soot filter of the invention, and thecatalysed soot filter of the invention is followed by (e.g. is upstreamof) the selective catalytic reduction (SCR) catalyst. Such anarrangement may be termed a NAC/CSF/SCR. A nitrogenous reductantinjector may be arranged between the catalysed soot filter and theselective catalytic reduction (SCR) catalyst. Thus, the catalysed sootfilter may be followed by (e.g. is upstream of) a nitrogenous reductantinjector, and the nitrogenous reductant injector may be followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.Alternatively, or additionally to the nitrogenous reductant injector,ammonia can be generated in situ e.g. during rich regeneration of a NACdisposed upstream of the filter.

In the seventh exhaust system embodiment with an SCR catalyst, theNO_(x) adsorber catalyst (NAC), the catalysed soot filter and theselective catalytic reduction (SCR) catalyst are generally connected inseries with an optional nitrogenous reductant injector being connectedbetween the oxidation catalyst and the selective catalytic reduction(SCR) catalyst. Thus, for example, an outlet of the NO_(x) adsorbercatalyst (NAC) is connected to an inlet of the catalysed soot filter,and outlet of the catalysed soot filter is connected to an inlet of theselective catalytic reduction (SCR) catalyst.

In any of the first through seventh inclusive exhaust system embodimentsdescribed hereinabove containing a SCR catalyst (including SCR catalysedfilters), an ASC catalyst can be disposed downstream from the SCRcatalyst or SCR catalysed filter (e.g. as a separate substratemonolith), or more preferably a zone on a downstream or trailing end ofthe substrate monolith comprising the SCR catalyst can be used as asupport for the ASC.

A third aspect of the invention relates to a compression ignition enginecomprising an exhaust system according to the second aspect of theinvention. The compression ignition engine can be a homogenous chargecompression ignition (HCCI) engine or a premixed charge compressionignition engine (PCCI) (see DieselNet Technology Guide “Engine Designfor Low Emissions”, Revision 2010.12a) or more conventional Port Fuelinjected-type compression ignition engines.

A fourth aspect of the invention relates to a vehicle comprising acompression ignition engine and the exhaust system for the compressionignition engine. Generally, the compression ignition engine is a dieselengine.

The vehicle may be a light-duty diesel vehicle (LDV), such as defined inUS or European legislation. A light-duty diesel vehicle typically has aweight of <2840 kg, more preferably a weight of <2610 kg.

In the US, a light-duty diesel vehicle (LDV) refers to a diesel vehiclehaving a gross weight of ≦8,500 pounds (US lbs). In Europe, the termlight-duty diesel vehicle (LDV) refers to (i) passenger vehiclescomprising no more than eight seats in addition to the driver's seat andhaving a maximum mass not exceeding 5 tonnes, and (ii) vehicles for thecarriage of goods having a maximum mass not exceeding 12 tonnes.

Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV),such as a diesel vehicle having a gross weight of >8,500 pounds (USlbs), as defined in US legislation.

A fifth aspect relates to the use of catalysed soot filter according tothe first aspect of the invention to oxidise carbon monoxide (CO) andhydrocarbons (HCs) and for reducing NO₂ emissions in an exhaust gas froma compression ignition engine; and the sixth aspect of the inventionrelates to a method of treating an exhaust gas from a compressionignition engine, which method comprises contacting the exhaust gas withthe catalysed soot filter according to the first aspect of theinvention.

Typically, the use and the corresponding method involve contacting theexhaust gas directly from the compression ignition engine with thecatalysed soot filter. Thus, it is preferred that additional hydrocarbon(HC) is generally not injected into the exhaust gas prior to contactingthe exhaust gas with the oxidation catalyst. The amount of hydrocarbonin the exhaust gas is preferably less than 1,000 ppm by volume, asconverted to methane, more preferably less than 950 ppm by volume, stillmore preferably less than 750 ppm, typically before contacting theexhaust gas with the oxidation catalyst.

DEFINITIONS

For the avoidance of doubt, the term “modified alumina incorporating aheteroatom component” does not embrace “pure” alumina (i.e. aluminahaving a purity of ≧99.9%), a mixture of alumina and the heteroatomcomponent, such as a mixture of silica and alumina, or a zeolite. In thecontext of the “modified alumina incorporating a heteroatom component”,any amount in % by weight refers to the amount of heteroatom component,whether an element, ion or a compound, that is present in the hostlattice of alumina with the remainder consisting essentially of alumina.

The term “alumina doped with a heteroatom component” generally refers toa material comprising a host lattice of alumina that is substitutiondoped or interstitially doped with a heteroatom component. In someinstances, small amounts of the heteroatom component may be present(i.e. as a dopant) at a surface of the alumina. However, most of thedopant will generally be present in the body of the host lattice of thealumina. Alumina doped with a heteroatom component is generallycommercially available, or can be prepared by conventional methods thatare well known in the art or by using a method as described in U.S. Pat.No. 5,045,519.

The term “alkaline earth metal component” as used herein generallyrefers to an element or ion from Group 2 of the Periodic Table, acompound comprising an element or ion from Group 2 of the PeriodicTable, or a metal alloy comprising an element from Group 2 of thePeriodic Table, unless otherwise specified. The term “alkaline earthmetal component” typically does not comprise or include the “modifiedalumina incorporating a heteroatom component”. The “alkaline earth metalcomponent” is not an “alumina doped with a heteroatom component” or an“alkaline earth metal aluminate” as described herein.

Generally, the “alkaline earth metal component” is (i) a compoundcomprising an alkaline earth metal, and/or (ii) a metal alloy comprisingan alkaline earth metal. In the compound comprising an alkaline earthmetal, the alkaline earth metal is typically present as a cation. Thecompound may, for example, be an alkaline earth metal oxide, an alkalineearth metal nitrate, an alkaline earth metal carbonate, or an alkalineearth metal hydroxide. In the metal alloy, the alkaline earth metal istypically present in elemental form (i.e. as a metal). The alkalineearth metal component is preferably a compound comprising an alkalineearth metal, more preferably a compound comprising a single alkalineearth metal.

The term “platinum group metal (PGM)” as used herein generally refers toplatinum or palladium, unless otherwise specified. For the avoidance ofdoubt, this term does not, in general, include rhodium.

The term “platinum group metal (PGM) component” as used herein refers toany moiety that comprises a platinum group metal (PGM), such aselemental PGM (e.g. a PGM metal), a PGM ion (e.g. a cation, such asPt²⁺), a compound comprising a PGM (e.g. a PGM salt or an oxide of aPGM) or an alloy comprising a PGM (e.g. a platinum-palladium alloy). Theterm “platinum (Pt) component” as used herein refers to any moiety thatcomprises platinum, such as elemental platinum (e.g. platinum metal), aplatinum ion (e.g. a cation, such as Pt²⁺), a compound of platinum (e.g.a platinum salt or an oxide of platinum) or an alloy comprising platinum(e.g. a platinum-palladium alloy). The term “palladium (Pd) component”as used herein refers to any moiety that comprises palladium, such aselemental palladium (e.g. palladium metal), a palladium ion (e.g. acation, such as Pd²⁺), a compound of palladium (e.g. a palladium salt oran oxide of palladium) or an alloy comprising palladium (e.g. aplatinum-palladium alloy).

The “platinum (Pt) component” is typically platinum metal or an alloycomprising platinum, particularly a platinum-palladium alloy.Preferably, the “platinum (Pt) component” is platinum metal.

The “palladium (Pd) component” is typically palladium metal, palladiumoxide or an alloy comprising palladium, particularly aplatinum-palladium alloy. Preferably, the “palladium (Pd) component” ispalladium metal.

The term “noble metal component” as used herein refers to any moietythat comprises a noble metal, such as an elemental form of the noblemetal, an ion of the noble metal, a compound of the noble metal or analloy comprising the noble metal (e.g. a noble metal-platinum alloy or anoble metal-palladium alloy). It is preferred that the “noble metalcomponent” is a noble metal itself (i.e. in elemental form) or an alloycomprising the noble metal. More preferably, the “noble metal component”is the noble metal itself (i.e. in elemental form).

Any reference to an amount of the “platinum group metal (PGM)component”, “platinum (Pt) component”, “palladium (Pd) component”,“alkaline earth metal component” or the “noble metal component” as usedherein generally refers to the amount of, respectively, PGM, platinum,palladium, alkaline earth metal or noble metal that is present. Thus,for example, if the “platinum group metal (PGM) component”, “platinum(Pt) component”, “palladium (Pd) component”, “alkaline earth metalcomponent” or the “noble metal component” is a compound comprising,respectively, a PGM, platinum, palladium, an alkaline earth metal or anoble metal, then the stated amount refers only to the total amount ofthe said metal that is present and does not include the other componentsof the compound.

Amounts given in units of g ft⁻³ or g in⁻³ generally relate to thevolume of the substrate that is used.

Any reference to an amount of a material in terms of % by weight, suchas for the amount of alkaline earth metal component, typically refers toa percentage of the overall weight of the layer/zone (e.g. washcoatcoating) comprising that material.

The term “substantially cover” as used herein refers to at least 90%coverage, preferably at least 95% coverage, more preferably at least 99%coverage, of the surface area of the first layer by the second layer.

The term “substantially uniform length” as used herein refers to thelength of layer that does not deviate by more than 10%, preferably doesnot deviate by more than 5%, more preferably does not deviate by morethan 1%, from the mean value of the length of the layer.

The expression “consisting essentially” as used herein limits the scopeof a feature to include the specified materials or steps, and any othermaterials or steps that do not materially affect the basiccharacteristics of that feature, such as for example minor impurities.The expression “consisting essentially of” embraces the expression“consisting of”.

The term “zone” as used herein refers to a washcoat region or layerhaving a length that is less than the total length of the substrate. Ingeneral, the “zone” has a substantially uniform length. The termnormally refers to the side-by-side arrangement of two or more washcoatregions or layers on the same substrate.

In order that the invention may be more fully understood, the followingExamples are provided by way of illustration only and with reference tothe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a histogram showing measurements of carbon monoxide“light-off” temperatures (° C.) at 50% conversion (CO T₅₀) for catalystformulations comprising varying amounts of platinum and palladium. Thex-axis shows the % by weight of platinum in relation to the totalcontent of platinum group metal in the formulation. At each point on thex-axis, the first bar (from the left) represents a sample containingstandard alumina and no barium, the second bar represents a samplecontaining alumina and barium, the third bar represents a samplecontaining a modified alumina and no barium, and the fourth barrepresents a sample containing a modified alumina and barium; and

FIG. 2 is a histogram showing measurements of carbon monoxide“light-off” temperatures (° C.) at 50% conversion (CO T₅₀) for catalystformulations according to Examples 6, 7 and 8 when tested on abench-mounted Euro 5 turbo-charged diesel engine running on <10 ppmsulphur fuel.

EXAMPLES Example 1 Preparative Methods

Samples containing an alkaline earth metal component and alumina dopedwith a heteroatom component as a support material were prepared asfollows.

Silica doped alumina powder was slurried in water and milled to a d₉₀<20micron. Barium acetate was added to the slurry followed by appropriateamounts of soluble platinum and palladium salts. The slurry was thenstirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C.

For comparative purposes, samples containing alumina doped with aheteroatom component as a support material, but without the alkalineearth metal component were also prepared. The method above was used toprepare the samples except that the step of adding barium acetate wasomitted.

As a further comparison, samples containing conventional alumina as asupport material with and without the alkaline earth metal componentwere prepared as follows.

Alumina powder was slurried in water and milled to a d₉₀<20 micron.Barium acetate was added to the slurry followed by appropriate amountsof soluble platinum and palladium salts. The slurry was then stirred tohomogenise. The resulting washcoat was applied to a cordierite flowthrough monolith having 400 cells per square inch using establishedcoating techniques. The part was dried and calcined at 500° C.

Analogous alumina samples without the alkali earth metal component wereprepared by omitting the barium acetate addition step.

The formulations contained a total loading of 50 gft⁻³ of platinum groupmetal. The samples containing barium were prepared using a loading of150 gft⁻³. Alumina doped with 5% silica was used as the modifiedalumina.

Measurement of CO T₅₀

The catalytic activity was determined using a synthetic gas benchactivity test (SCAT). Parts to be tested were first cored using a coredrill and aged in an oven at 750° C. for 5 hours using hydrothermalconditions (10% water). The aged cores were tested in a simulatedcatalyst activity testing (SCAT) gas apparatus using the inlet gasmixtures in Table 1. In each case the balance is nitrogen.

TABLE 1 Gas component Composition CO 1500 ppm  HC (as C₁) 430 ppm NO 100ppm CO₂ 4% H₂O 4% O₂ 14%  Space velocity 55000/hour

Results

The results of the measurements are shown in FIG. 1. FIG. 1 shows theimproved activity of catalysts of the invention, which comprise both amodified alumina and barium at a loading of 150 gft⁻³. The catalysts ofthe invention have a lower CO T₅₀ light off temperature than thecomparative catalysts that do not contain barium. This is particularlyapparent when the ratio by mass of Pt:Pd is in the range of 1:2 to 2:1.The catalysts containing conventional alumina as a support material andbarium do not show an improved CO T₅₀ light off temperature compared tocatalysts containing conventional alumina and no barium.

Example 2

Samples containing alumina doped with 5% silica, Pt:Pd in a mass ratioof 1:1 at a total PGM loading of 50 gft⁻³ and varying amounts of bariumwere prepared using the method described above. The CO T₅₀ light offtemperatures were also measured using the same procedure as set outabove.

Results

The results of the CO “light-off” measurements are shown in Table 2below.

TABLE 2 Amount of CO T₅₀ Sample No. Ba (gft⁻³) (° C.) 2-1 0 186 2-2 150170 2-3 300 166

Example 3 Preparative Method

Silica doped alumina powder was slurried in water and milled to a d₉₀<20micron. Strontium acetate was added to the slurry followed byappropriate amounts of soluble platinum and palladium salts. The massratio of Pt:Pd was 1:1 at a total PGM loading of 50 gft⁻³. The slurrywas then stirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C. The CO T₅₀ light off temperatures were also measured using the sameprocedure as set out above.

Results

The results of the CO “light-off” measurements are shown in Table 3below.

TABLE 3 Alkaline Earth Amount of CO T₅₀ Sample No. Metal (AEM) AEM(gft⁻³) Support (° C.) 3-1 Ba 100 A1 164 3-2 Ba 150 A2 166 3-3 Sr 150 A1178 3-4 Sr 300 A1 176

Catalysts containing strontium show a lower CO T₅₀ light off than acomparative catalyst without strontium (see Sample 2-1 in Table 2).Samples 3-1 and 3-2 show that a reduction in CO T₅₀ light off isachieved using two different hetero-atom doped aluminas Support A1 is a5% silica doped alumina and support A2 is a 10% silica doped alumina.

Example 4 Preparative Method

Magnesium doped alumina powder was slurried in water and milled to ad₉₀<20 micron. Barium acetate was added to the slurry followed byappropriate amounts of soluble platinum and palladium salts. The massratio of Pt:Pd was 2:1 at a total PGM loading of 50 gft⁻³. The slurrywas then stirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C.

For comparative purposes, catalysts containing magnesium doped aluminaas a support material without an alkali earth metal were prepared in thesame way, except that the barium acetate addition step was omitted.

The CO T₅₀ light off temperatures were also measured using the procedureset out above.

Results

The results of the CO “light-off” measurements are shown in Table 4below.

TABLE 4 Amount of CO T₅₀ Sample No. Ba (gft⁻³) Support (° C.) 4-1 0 A1177 4-2 130 A1 170 4-3 0 A3 193 4-4 130 A3 174 A3 = Magnesium alummatehaving 20% by weight magnesium.

The samples containing a support material comprising magnesium aluminateshow a lower CO T₅₀ light off when barium is included in the formulationthan those sample prepared without barium. The light off temperature isreduced by 19° C.

Example 5 Preparative Method

A catalyst (5-1) was prepared via incipient wetness impregnation ofbarium acetate solution on to a silica doped alumina support. Thematerial was dried at 105° C. A second solution of platinum andpalladium salts was then added by incipient wetness impregnation. Theresulting material was dried at 105° C. then calcined at 500° C. Thefinal composition was 0.65 wt % Pt, 0.35 wt % Pd and 10 wt % Ba.

A comparative catalyst (5-2) was prepared using the same method butwithout the barium acetate impregnation on to the silica doped aluminasupport. The final composition was 0.65 wt % Pt and 0.35 wt % Pd.

The % NO oxidation activity against temperature of each catalyst wastested when the catalyst was freshly prepared (e.g. “fresh” catalyst)and after hydrothermal ageing each catalyst at 750° C. for 48 hours(e.g. “aged” catalyst). The test gas mix is given in Table 5. In eachcase the balance is nitrogen.

TABLE 5 Gas component Composition CO 1500 ppm  HC (as C₁) 783 ppm NO 100ppm CO₂ 5% H₂O 5% O₂ 14% 

Results

The difference between the activity of the “fresh” and the “aged”versions of each catalyst is shown in Table 6 below.

TABLE 6 Difference in % NO oxidation between “fresh” and “aged” SampleNo. 210° C. 270° C. 5-1  3%  4% 5-2 21% 23%

The results in Table 6 show that catalyst 5-1 has a smaller differencein % NO oxidation performance between “fresh” and “aged” than catalyst5-2. This difference is important for exhaust system where there is adownstream emissions control device, particularly a SCR or SCRFcatalyst, because the activity of such downstream emissions controldevices may be affected by the NO_(x) content of the exhaust gas,especially the NO:NO₂ ratio.

Example 6

Alumina powder was slurried in water and milled to a particle size d90<6μm. Appropriate amounts of soluble platinum and palladium salts wereadded to the slurry, followed by beta zeolite such that the slurrycomprised 80% alumina and 20% zeolite by mass. The slurry was stirred tohomogenise. The resulting washcoat was applied to a 3.0 liter volumesilicon carbide substrate having 300 cells per square inch (cpsi), 12thousandths of an inch wall thickness and 42% porosity using establishedcoating techniques. The part was dried and calcined at 500° C.

The finished catalysed soot filter had a total PGM loading of 20 g ft⁻³.

Example 7

Alumina powder was slurried in water and milled to a particle size d90<6μm. Barium acetate was added to the slurry followed by appropriateamounts of soluble platinum and palladium salts and beta zeolite suchthat the slurry comprised 80% alumina and 20% zeolite by mass. Theslurry was stirred to homogenise. The resulting washcoat was applied toa 3.0 liter volume silicon carbide substrate having 300 cpsi, 12thousandths of an inch wall thickness and 42% porosity using establishedcoating techniques. The part was dried and calcined at 500° C.

The finished catalysed soot filter had a total PGM loading of 20 g ft⁻³.

Example 8

Silica doped alumina powder was slurried in water and milled to aparticle size d90<6 μm. Barium acetate was added to the slurry followedby appropriate amounts of soluble platinum and palladium salts and betazeolite such that the slurry comprised 80% silica doped alumina and 20%zeolite by mass. The slurry was stirred to homogenise. The resultingwashcoat was applied to a 3.0 liter volume silicon carbide substratehaving 300 cpsi, 12 thousandths of an inch wall thickness and 42%porosity using established coating techniques. The part was dried andcalcined at 500° C.

The finished catalysed soot filter had a total PGM loading of 20 g ft⁻³.

Example 9 Testing

The samples prepared according to Examples 6, 7 and 8 were eachhydrothermally aged in an oven at 800° C. for 16 hours. They were thenexposed to exhaust gas emissions from a Euro 5 turbo charged dieselbench mounted engine running <10 ppm sulphur fuel. Pollutant emissionswere measured both before and after the catalysed soot filters. Thelight-off activity of the catalysts was determined by increasing theexhaust gas temperature by applying load to the engine via adynamometer.

FIG. 2 shows the T₅₀ CO light off activity of Example 6, Example 7 andExample 8. In FIG. 2 we see that the sample of Example 6 and Example 8have very similar light-off temperatures. However, the sample of Example7 has a much higher CO light off.

Table 7 shows the percentage of NO₂ in NO_(x) emitted from the catalysedsoot filter at 300° C. during the light-off test. Example 6 has arelatively high percentage of NO₂ in NO_(x) at 15%. Example 7 andExample 8 both show just 1% NO₂ in NO_(x).

TABLE 7 NO₂ in NO_(x) at 300° C. (Emissions from bench-mounted Euro 5turbo-charged diesel engine running on <10 ppm sulphur fuel) NO₂ inNO_(x) Example 6 15%  Example 7 1% Example 8 1%

Example 8 according to the invention shows low NO₂ in NO_(x) whilstmaintaining good CO oxidation activity.

For the avoidance of any doubt, the entire content of any and alldocuments cited herein is incorporated by reference into the presentapplication.

The invention claimed is:
 1. A catalysed soot filter comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from a compression ignition engine disposed on a filtering substrate, wherein the oxidation catalyst comprises a plurality of layers on the filtering substrate, wherein a layer comprises: a platinum group metal (PGM) component selected from the group consisting of a platinum (Pt) component, a palladium (Pd) component and a combination thereof; an alkaline earth metal component; and a support material comprising a modified alumina incorporating a heteroatom component.
 2. A catalysed soot filter according to claim 1, wherein the total amount of the alkaline earth metal component is 10 to 500 gft⁻³.
 3. A catalysed soot filter according to claim 1, wherein the filtering substrate comprises a plurality of inlet channels and a plurality of outlet channels; the oxidation catalyst comprises a single layer on the inlet channels and a single layer on the outlet channels; and wherein the single layer on the inlet channels comprises the platinum group metal (PGM) component, the alkaline earth metal component and the support material.
 4. A catalysed soot filter according to claim 1, wherein the filtering substrate comprises inlet channels and outlet channels; the oxidation catalyst comprises a single layer on the inlet channels and a single layer on the outlet channels; and wherein the single layer on the outlet channels comprises the platinum group metal (PGM) component, the alkaline earth metal component and the support material.
 5. A catalysed soot filter according to claim 1, wherein the oxidation catalyst comprises two layers, wherein a first layer is disposed on the filtering substrate and a second layer is disposed on the first layer, and wherein the first layer comprises the platinum group metal (PGM) component, the alkaline earth metal component and the support material.
 6. A catalysed soot filter according to claim 1, wherein the oxidation catalyst comprises two layers, wherein a first layer is disposed on the filtering substrate and a second layer is disposed on the first layer, and wherein the second layer comprises the platinum group metal (PGM) component, the alkaline earth metal component and the support material.
 7. A catalysed soot filter according to claim 1, wherein the modified alumina incorporating a heteroatom component is an alumina doped with a heteroatom component, an alkaline earth metal aluminate or a mixture thereof.
 8. A catalysed soot filter according to claim 1, wherein the heteroatom component comprises silicon, magnesium, barium, lanthanum, cerium, titanium, zirconium or a combination of two or more thereof.
 9. A catalysed soot filter according to claim 1, wherein the modified alumina incorporating a heteroatom component is alumina doped with silica.
 10. A catalysed soot filter according to claim 9, wherein the alumina is doped with silica in an amount of 0.5 to 45% by weight.
 11. A catalysed soot filter according to claim 1, wherein the modified alumina incorporating a heteroatom component is an alkaline earth metal aluminate.
 12. A catalysed soot filter according to claim 11, wherein the alkaline earth metal aluminate is magnesium aluminate.
 13. A catalysed soot filter according to claim 1, wherein the alkaline earth metal component comprises magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) or a combination of two or more thereof.
 14. A catalysed soot filter according to claim 13, wherein the alkaline earth metal component comprises strontium (Sr) or barium (Ba).
 15. A catalysed soot filter according to claim 1, wherein the filtering substrate is a ceramic wallflow filter.
 16. An exhaust system for a compression ignition engine comprising a catalysed soot filter according to claim
 1. 17. An exhaust system according to claim 16, comprising a diesel oxidation catalyst disposed on a separate flow-through substrate monolith, which is disposed upstream of the catalysed soot filter.
 18. A compression ignition engine comprising an exhaust system according to claim
 16. 19. A method of treating carbon monoxide (CO) and hydrocarbons (HCs) and NO₂ in an exhaust gas from a compression ignition engine, which method comprises contacting the exhaust gas with a catalysed soot filter according to claim
 1. 